Dependent Amino Acid Transport System Research Articles

Reversed transport of amino acids in Ehrlich cells

Gramicidin induces a marked Na +-dependent efflux of amino acids from Ehrlich cells. In absence of Na +, gramicidin does not alter the efflux. In presence of gramicidin, glycine efflux is inhibited by methionine and less so by leucine. Glycine efflux caused by HgCl 2 is neither Na + dependent nor inhibitable by amino acids. Neither efflux of inositol which is transported by an Na +-dependent route, nor efflux of several other solutes which are transported by Na +-independent routes, is affected by gramicidin. The antibiotic appears to permit a reversal in the direction of the operation of the Na +-dependent amino acid transport system. The increased efflux is partly, but not entirely, due to an increase in the cellular Na + concentration and a reduction of the electrochemical potential difference for Na +.

Cation flux in the ehrlich ascites tumor cell evidence for Na +-for-Na + and K +-for-K + exchange diffusion

In a previous study, evidence was presented for an external Na +-dependent, ouabain-insensitive component of Na + efflux and an external K +-dependent component of K + efflux in the Ehrlich ascites tumor cell. Evidence is now presented that these components are inhibited by the diuretic furosemide and that under conditions of normal extracellular Na + and K + they represent Na +-for-Na + and K- +for-K + exchange mechanisms. Using 86Rb to monitor K + movements, furosemide is shown to inhibit an ouabain-insensitive component of Rb + influx and a component of Rb + efflux, both representing approx. 30% of the total fux. Inhibition of Rb + efflux is greatly reduced by removal of extracellular K +. Furosemide does not alter steady-state levels of intracellular K + and it does not prevent cells depleted of K + by incubation in the cold from regaining K + upon warming. Using 22Na to monitor Na + movements, furosemide is shown to inhibit an ouabain-insensitive component of unidirectional Na + efflux which represents approx. 22% of total Na + efflux. Furosemide does not alter steady-state levels of intracellular Na + and does not prevent removal of intracellular Na + upon warming from cells loaded with Na + by preincubation in the cold. The ability of furosemide to affect unidirectional Na + and K + fluxes but not net fluxes is consistent with the conclusion that these components of cation movement across the cell membrane represent one-for-one exchange mechanisms. Data are also presented which demonstrate that the uptake of α-aminoisobutyrate is not affected by furosemide. This indicates that these components of cation flux are not directly involved in the Na +-dependent amino acid transport system A.

Activation of a Na +-dependent amino acid transport system in preimplantation mouse embryos

The uptake of l-methionine-methyl- 3H and l-leucine- 3H from completely defined medium into acid-soluble fractions of preimplantation mouse embryos has been studied. Late four-cell embryos and early blastocysts raised in vitro can concentrate both amino acids by processes which exhibit saturable, Michaelis-Menten type kinetics, characteristic of carrier-mediated active transport systems. This uptake is temperature-sensitive and inhibited by certain amino acids which compete for the same uptake sites. Methionine uptake seems to be mediated by a single transport system ( K m = 6.25 × 10 −5 M) at the four-cell stage. Complex kinetics suggest that two distinct transport systems exist at the early blastocyst stage ( K m = 6.25 × 10 −5 M; 8.9 × 10 −4 M). V max values (mg/embryo/15 min) for methionine and leucine transport increase significantly from the late four-cell stage to the blastocyst stage, suggesting that additional carriers are produced or activated during development. Most importantly, leucine and methionine transport is Na +-independent at the four-cell stage, methionine transport is partially dependent at the morula stage, and both amino acids are completely Na +-dependent at the blastocyst stage. The cumulative results suggest that preimplantation embryos accumulate leucine and methionine by specific, chemically mediated, active transport systems. The qualitative and quantitative developmental changes in cell membrane function may represent preparatory steps for subsequent growth of embryonic and/or trophoblastic cells.

Selective early activation of a sodium dependent amino acid transport system in stimulated rat lymphocytes

Chemicals/CAS: 2 amino 2 methylpropionic acid, 62-57-7; amino acid, 65072-01-7; concanavalin A, 11028-71-0; sodium, 7440-23-5; Amino Acids; Aminoisobutyric Acids; Concanavalin A, 11028-71-0; Cyclopentanes; Sodium, 7440-23-5

Activation of an Na +-dependent amino acid transport system upon fertilization of sea urchin eggs

Unfertilized eggs possess a Na +-independent amino acid transport system, capable of concentrating lysine, valine and phenylalanine but not glycine, alanine or serine. An Na +-dependent transport system for all the above amino acids begins to be formed at 6–10 min after fertilization. Inhibitors of protein synthesis have no effect on its formation, indicating that increased transport results from an activation or alteration of a pre-existing system. A product of energy metabolism is required, since activation does not occur if 2,4-dinitrophenol (DNP) is added between 0 and 6 min after fertilization. This is not an effect of DNP on transport, since transport is unaffected if DNP is added after the system is fully formed. A relationship is suggested between the activation of Na +-dependent transport and the establishment of the K +-sensitive membrane potential.